An insulated gate bipolar transistor (IGBT) is a semiconductor power device with a compositing structure that combines a metal-oxide-semiconductor field effect transistor (MOSFET) and a bipolar junction transistor (BJT). Performance features of an IGBT are designed to achieve a higher current density than the MOSFET's, and faster and more efficient switching characteristics and better control than the BJT's. Additionally, the drift region of the IGBT can be lightly doped for improved blocking ability. Meanwhile, the device can still have good conductivity because the lightly doped drift region undergoes high level carrier injection from a bottom P collector region resulting in conductivity modulation. With the MOSFET's characteristic of easy control with a gate electrode, the bipolar current flow mechanism and the advantages of shorter switching time and lower power loss, the IGBT is widely applied in a high voltage and high power application.
Conventional technologies to configure and manufacture IGBT devices are still confronted with difficulties and limitations to further improvement in performance due to various tradeoffs. In IGBT devices, there is a tradeoff between conduction loss and turn-off switching losses, Eoff. Conduction loss depends upon the collector to emitter saturation voltage Vce(SAT) at rated current. More carrier injection while the device is on improves the conductivity of the device, thus reducing conduction loss. However, more carrier injection would also cause higher turn-off switching losses because of the energy dissipated in clearing out the injected carriers during turn-off.
Another trade-off exists between the IGBT's collector-emitter voltage at saturation (Vce(SAT)) and its breakdown voltage (VBD). While an increase on topside injection may improve Vce(SAT), it usually comes at a cost of lowering breakdown voltage VBD. An IGBT device with a high density deep trench may overcome this trade-off, but it is hard to make such device with a high density of small pitch high aspect ratio trenches.
There are different configurations of IGBT devices, such as planar gate IGBT devices and IGBT device of the trench gate type. FIG. 1A is a cross sectional view of a conventional planar gate IGBT. FIG. 1B is a cross sectional view of another conventional IGBT device that has a trench gate. Both configurations of FIGS. 1A and 1B include a second gate G2 disposed over a p-type well region (20 or 120) to form a MOSFET channel between the first well region (22 or 122) and the drift region (24 or 124). Since the p-type well region 20 (also similar for p-type well region 120) has a P region 20a in the main current path and a region 20b extends upwardly to the surface 18 of the structure, it makes the fabrication process complicated. In addition, for the planar gate IGBT device, the second gate G2 wastes active device area.
It is within this context that embodiments of the present invention arise.